The invention relates to a method for loading an electronic amount of money represented by a random number sequence, to a portable data carrier, and a method for disbursing an amount of money from the data carrier.
Prepaid cards are known for loading an electronic amount of money to a credit account for mobile telephones. After payment of the amount of money to the trader, the buyer is handed the prepaid card. The buyer uncovers a credit code applied in covered fashion to the prepaid card and transmits the credit code to his mobile communication network operator, stating his mobile telephone number. The network operator tops up the credit account associated with the mobile telephone number by the amount of money.
Electronic wallets (e-wallet, electronic purse) are known for paying small amounts of money. There exist electronic wallets implemented in chip cards, such as e.g. the German cash card. Further, electronic wallets are known which are implemented as applications in mobile terminals, such as e.g. mobile telephones, smart phones, PDAs and the like.
A typical electronic wallet, e.g. the German cash card, has a dual structure, with a local wallet account in a portable data carrier, e.g. in a chip card or in a chip in the mobile terminal, and a shadow account on a background server. To credit an amount of money to the wallet, a payment is effected to the background server, e.g. in cash or with a payment transaction card. In response to the payment, the paid amount is credited to the shadow account and the wallet account. When an amount of money is paid with the wallet, the amount of money is first debited from the wallet account and subsequently, possibly also much later, the shadow account is adjusted, i.e. the amount of money is debited also from the shadow account.
There is a plurality of micro payment systems for paying (predominantly) small electronic amounts of money.
In the so-called Bitmint concept for a micro payment system, a buyer obtains a character sequence (bit string) in exchange for the payment of an amount of money, the character sequence corresponding to the amount of money. An amount of money of x cents is represented in Bitmint by a random number of a length of x*N bytes, e.g. with N=32 bytes. FIG. 3 schematically shows an implementation possibility for a typical Bitmint character sequence. In the implementation possibility suggested in FIG. 3, the amount of money is represented as a sequence of random numbers. The smallest available unit 1 cent is represented by a random number of a length of N (e.g. 32) bytes. An amount of money of x cents is represented by a sequence of random numbers RND which consists of a concatenation of the x arrayed random numbers RND0, RND1, . . . , RNDx of the x individual cents, the random numbers having a length of N bytes. Besides the sequence of random numbers RND which represents the amount of money, the character sequence comprises a position field POS, where the start position POSS and length POSL of the random number RND are specified. By means of the position field POS, the random number can be divided in order to divide the total amount into partial amounts, in order to return change for example. The total amount is specified by the start position POSS at the start of the random number sequence RND and the length POSL total length of the random number sequence. Partial amounts are represented by selected values of the position POSS and the length POSL. Optionally, the character sequence comprises an index IN facilitating administration, and an attribute field AT.
It would be desirable to be able to carry along electronic Bitmint money in an electronic wallet. In order to store a larger amount of money in Bitmint, a multiplicity of long random numbers must be stored. The typically available memory space of a chip card or of a wallet application in a mobile terminal is not sufficient for this.